This invention relates to a steering mechanism and more particularly, to an adjustable Ackerman steering mechanism having an adjustable steering knuckle arm and tie rod to operate with various wheelbase lengths of a motor vehicle.
The steering mechanism on an Ackerman steering system turns the two steerable wheels of a motor vehicle so that all the wheels roll on circles with a common center. If the vehicle is to turn without lateral skid of any wheel, the center lines of all wheel axles must intersect when extended at every instant in a common center about which the vehicle turns. It is used universally on wheeled vehicles. For straight forward motion the front wheels are substantially parallel but as the vehicle enters a curve, the inner wheel turns more sharply than the outer wheel. The extreme condition occurs when the vehicle is on a curve of its minimum turning radius. A common configuration that produces Ackerman steering inclines the steering knuckle arms inwardly and rearwardly. The angle of the inclination depends on the wheelbase and the tread of the vehicle. Wheelbase is the distance from the front to the rear wheels measured between the centers of ground contact. Tread is the distance from the left wheel to the right wheel measured between the centers of ground contact.
To roll smoothly when a vehicle is directed at the desired course, the steerable wheels are mounted at a slight angle to the normal. As viewed from the driver's position, the wheel tilts outwardly from the vertical. This tilt is called camber and is positive if outwardly as shown in FIG. 3a. The wheels are given slight positive camber usually less than a degree initially, so that when the vehicle is loaded, the wheels become almost vertical.
The kingpin or axis of the pivot for the wheels is tilted slightly inward. The kingpin inclination causes the chassis to rise slightly when the wheels are turned. Thus the weight of the vehicle tends to keep the wheels turned straight ahead or to return it to that position after a turn. This self return of a steering system to the straight line travel is called recovery.
Camber and kingpin inclination together are called the included angle. If the apex of this angle lies on the road surface road resistance on the tire and forward push on the kingpin are along the line of roll. If the apex is not on the road, the tire is forced to toe in or out. To take up play in the front wheel supports, the front wheels are purposely toed in slightly so that the planes of the wheels intersect ahead of the vehicle. Road resistance then forces them to roll parallel and takes up any play in the steering system. The inclination of the kingpin or its equivalent forward or backward is called caster. Caster is positive if the kingpin inclination is backward, negative if it inclines forward and zero if the kingpin is vertical as viewed along the line of the front wheels. Positive caster aids in directional stability and in recovery.
The wheelbase on a motor vehicle varies depending on the size and the length of the vehicle. Although automobiles generally have a fixed wheelbase for a given model, highway trucks are frequently manufactured with different length wheelbases in a given model line depending on the type of body and the use to which the vehicle will be put. The differential rate of steering of the steerable wheels is varied with a variation in the wheelbase. With a longer wheelbase, the differential rate of steering of the steerable wheels is less than with a short wheelbase. Adjustment to vary the rate of differential steering, in other words, the degree of toe out of the outside wheels relative to the inside wheel, is varied. The differential rate of steering and the degree of toe out can be controlled with the angle of the steering knuckle arm relative to the wheel spindle. Also, the tie rod length is varied when the angle of the steering knuckle arm is varied. When the motor vehicle is manufactured, these relative angles are taken into consideration. Accordingly the present invention deals with a single steering assembly which is adjustable to accommodate a selected one of a range of wheelbase lengths. Optimum steering geometry is provided by adjusting the steering knuckle arm and the tie rod.